1. Field of the Invention
This invention relates to a feedforward amplifier that, in a distortion detection loop, uses a main amplifier to amplify a plurality of signal groups wherein each has a different frequency band, and in a distortion compensation loop, also compensates for the distortion that arises in said main amplifier and particularly to a feedforward amplifier that is able to perform precise distortion compensation on a plurality of wideband signal groups as a whole without adopting broadband frequency-band characteristics for the auxiliary amplifier used to amplify the distortion detected by the distortion detection loop.
2. Description of the Prior Art
For example, in base station units and relay station units used for mobile communications, a multi-carrier signal that contains a plurality of carriers at stipulated frequency intervals that are each modulated appropriately is transmitted wirelessly after high-frequency amplification. Unless the amplifier used for high-frequency amplification has sufficient linearity, various kinds of distortion including intermodulation distortion and the like may occur. This distortion becomes an impediment to achieving normal and high-quality communication. For this reason, the amplifiers used for the amplification of multi-carrier signals are required to have good linearity over the entire frequency band to which the multi-carrier signal belongs.
One example of a method of achieving an ultra-low-distortion amplifier suited to the amplification of a multi-carrier signal is the feedforward (FF) amplification scheme.
Here in this Specification, within the FF amplification scheme, the signal path from the signal input terminal via the main amplifier to the signal output terminal, namely the signal path for conveying the signal to be amplified and the amplified signal is referred to as the xe2x80x9cmain line.xe2x80x9d
In addition, in the FF amplification scheme, a distortion detection path that couples a signal branching from a point after the main amplifier in the main line to a signal branching from a point before the main amplifier in the main line. In this Specification, the loop consisting of the main line and the distortion detection path is called the distortion detection loop.
Moreover, in the distortion detection loop, the electrical lengths of the signal paths over which both the signal conveyed along the main line and the signal conveyed along the distortion detection path are equal to each other, and, if both signals have mutually opposite phases at the same amplitude, then with the aforementioned signal coupling action, it is possible to cancel the carrier-wave components and extract a signal equivalent to the distortion arising in the main amplifier and its peripheral circuits.
Moreover, in the FF amplification scheme, a distortion compensation path is provided to recouple the signal extracted by the distortion detection loop, namely a signal equivalent to the distortion, to the signal on the main line. In this Specification, the loop consisting of the main line and the distortion compensation loop is called the distortion compensation loop.
Moreover, signal delay in the distortion compensation loop is compensated for in the main line and as long as adjustment of the amplitude and phase is performed appropriately in the distortion compensation loop so that the distortion components contained in the signal on the main line have the same amplitude and the opposite phase from the distortion signal obtained from the distortion compensation loop, it is possible to compensate for the distortion occurring in the main amplifier.
FIG. 3 shows an example of the constitution of a conventional FF amplifier.
In the FF amplifier shown in the figure, three directional couplers (hybrid) 41, 46 and 51 are utilized to form a distortion detection loop L11 consisting of the main line and the distortion detection path and a distortion compensation loop L12 consisting of the main line and the distortion compensation path.
Specifically, the signal path from the signal input terminal P2 via the first directional coupler 41, first variable attenuator 42, first variable phase shifter 43, first coupler 61, main amplifier (first amplifier) 44, second directional coupler 46, second coaxial delay line 47, third directional coupler 51 and third coupler 63 to the signal output terminal Q2 is equivalent to the main line.
In addition, the signal path from the signal input terminal P2 via the first directional coupler 41 and first coaxial delay line 45 to the output terminal of the second directional coupler 46 is equivalent to the distortion detection path.
In addition, the signal path from the output terminal of the second directional coupler 46 via the second coupler 62, second variable attenuator 48, second variable phase shifter 49 and auxiliary amplifier (second amplifier) 50 to the output terminal of the third directional coupler 51 is equivalent to the distortion compensation path.
Note that the terminating resistors R11 and R12 are connected to one input terminal of the first directional coupler 41 and one output terminal of the third directional coupler 51 as terminating dummy loads. These terminating resistors R11 and R12 have an impedance of Z0 equal to the characteristic impedance of the path.
In addition, with the FF amplifier shown in the figure, the three couplers 61, 62 and 63 and a control circuit 71 which has a control signal generation circuit 72 constitute the feedback control system which performs control using pilot signals and the like.
Here follows one example of the operation performed by the FF amplifier shown in the figure.
For example, when a multi-carrier signal is applied to the signal input terminal P2, this signal is input via the first directional coupler 41 to the first variable attenuator 42 and first variable phase shifter 43, where its amplitude and phase are adjusted, and then further input to the main amplifier 44 where it is amplified. The signal amplified by the main amplifier 44 is input via the second directional coupler 46 and second coaxial delay line 47 to the third directional coupler 51, and further output from the third directional coupler 51 via the signal output terminal Q2 to the circuits after this FF amplifier. Note that the second coaxial delay line 47 is a delay line that compensates for the signal delay arising in the circuit constituting the distortion compensation loop L12, e.g., the auxiliary amplifier 50.
In addition, the signal input from the signal input terminal P2 is divided into two branches by the first directional coupler 41. The two branches of the signal are the same signal from the standpoint of the constitution of component frequencies, and the signal branch supplied to the main line side is amplified by the main amplifier 44, while the signal branch supplied to the distortion detection path side is supplied at roughly its original amplitude from the first directional coupler 41 via the first coaxial delay line 45 to the second directional coupler 46. Note that the first coaxial delay line 45 is a delay line for compensating for the signal delay arising in the main amplifier 44 in particular.
The signal supplied via the first coaxial delay line 45 to the second directional coupler 46 is coupled by this second directional coupler 46 to the amplified signal that contains distortion components.
Specifically, the second directional coupler 46 divides the signal containing distortion components output from the main amplifier 44 into two branches. The two branches of the signal are the same signal from the standpoint of the constitution of component frequencies; one signal branch is supplied to the main line side while the other signal branch is supplied to the distortion compensation path side When this other signal branch is supplied to the distortion compensation path side, the second directional coupler 46 couples this other signal branch to the signal input via the first coaxial delay line 45, thereby canceling the carrier-wave components within the other signal branch and extracting the distortion components from this other signal branch.
The distortion signal obtained as a result of this coupling is supplied from the second directional coupler 46 to the second variable attenuator 48, second variable phase shifter 49 and auxiliary amplifier 50 which constitute the distortion compensation path, its amplitude and phase are adjusted in the second variable attenuator 48 and second variable phase shifter 49, and moreover it is amplified by the auxiliary amplifier 50 and provided as input to the third directional coupler 51. The distortion signal input to the third directional coupler 51 is coupled in the third directional coupler 51 to the signal input via the second coaxial delay line 47, the distortion contained within this signal is cancelled and this signal with the distortion cancelled is output from signal output terminal Q2.
Here, in order to extract the distortion arising in the main amplifier 44 or the like by coupling the signal branch of the output signal from the main amplifier 44 to the signal traveling via the first coaxial delay line 45 in the second directional coupler 46, a stipulated number of carrier-wave components contained in the signal branch of the output signal from the main amplifier 44 and the same number of carrier-wave components contained in the signal via the first coaxial delay line 45 are necessary at the time of coupling in the second directional coupler 46 at the same timing, same amplitude and opposite phase from each other.
The first coaxial delay line 45 is a means of applying the delay time D1 to the signal and synchronizing the timing among the carrier-wave components. In addition, the first variable attenuator 42, first variable phase shifter 43 and the control circuit 71 which exerts control so as to adjust the signal attenuation G1 in the first variable attenuator 42 and the signal phase shift xcex81 in the first variable phase shifter 43 to optimal values are the means of bringing the carrier wave components to the same amplitude and opposite phase.
In this manner, the first coaxial delay line 45, first variable attenuator 42, first variable phase shifter 43 and control circuit 71 are means of adjusting the output from the second directional coupler 46 so that a signal containing only distortion components and containing no carrier-wave components is supplied to the auxiliary amplifier 50.
In addition, in order to compensate for distortion by coupling in the third directional coupler 51 the signal coming via the second coaxial delay line 47 to the signal coming via the auxiliary amplifier 50, it is preferable that, firstly, the signal coming via the auxiliary amplifier 50 be a signal that contains only distortion components and contains no carrier-wave components. Here, the generation of distortion in the auxiliary amplifier 50 is negligible when the distortion detection loop L11 is operating normally, so it is possible to set an appropriate value for the delay time D1 of the first coaxial delay line 45 and also have the control circuit 71 appropriately control the first variable attenuator 42 and first variable phase shifter 43 to satisfy this first condition.
In addition, in order to compensate for distortion in the third directional coupler 51, secondly, the distortion components within the signal coming via the second coaxial delay line 47 and the distortion components within the signal coming via the auxiliary amplifier 50 are necessary at the time of coupling in the third directional coupler 51 at the same timing, same amplitude and opposite phase from each other.
The second coaxial delay line 47 is a means of applying the delay time D2 to the signal and synchronizing the timing among the carrier-wave components. In addition, the second variable attenuator 48, second variable phase shifter 49, and the control circuit 71 which exerts control so as to adjust the signal attenuation G2 in the second variable attenuator 48 and the signal phase shift xcex82 in the second variable phase shifter 49 to optimal values are the means of bringing the carrier wave components to the same amplitude and opposite phase.
Note that the process of exerting control so as to adjust the first signal attenuation G1 and first signal phase shift xcex81 and the second signal attenuation G2 and signal phase shift xcex82 to optimal values, namely the process of optimizing the state of the distortion detection loop L11 and distortion compensation loop L12 is executed by the control circuit 71. In the FF amplifier shown in the figure, this optimization process is executed by the control circuit 71 which operates under the control of a central processing unit (CPU).
Specifically, the second coupler 62 detects a portion of the output of the second directional coupler 46 to the distortion compensation path, and outputs the results of this detection to the control signal generation circuit 72 of the control circuit 71. Then, the control signal generation circuit 72 controls the first signal attenuation G1 of the first variable attenuator 42 and the first signal phase shift xcex81 of the first variable phase shifter 43 so that only the distortion generated by the second coupler 62 is detected.
In addition, the control signal generation circuit 72 of the control circuit 71 generates a stipulated pilot signal and outputs this pilot signal to the first coupler 61. The first coupler 61 combines this pilot signal with the output from the first variable phase shifter 43 and outputs this composite signal to the main amplifier 44. In addition, the third coupler 63 detects the components of the pilot signal inserted into the input signal subject to amplification with the first coupler 61 and outputs the results of this detection to the control signal generation circuit 72 of the control circuit 71. Based on the results of detection input from the third coupler 63, the control signal generation circuit 72 controls the signal attenuation G2 of the second variable attenuator 48 and the signal phase shift xcex82 of the second variable phase shifter 49, thereby achieving good compensation of distortion.
However, with the conventional FF amplifier as shown in FIG. 3 above, when processing a group of signals having a plurality of different frequency bands such as in a multi-carrier signal, there was a problem in that it was necessary to adopt broadband frequency-band characteristics for the auxiliary amplifier 50. Specifically, it is difficult to obtain a flat broadband response for the amplification characteristics and phase characteristics at the time of amplification with the auxiliary amplifier 50, for example.
The present invention came about in light of the aforementioned circumstances and has as its object to provide a feedforward amplifier that has a distortion detection loop and a distortion compensation loop, and that uses a main amplifier to amplify a plurality of signal groups wherein each has a different frequency band, and in a constitution wherein the distortion arising in this main amplifier is compensated for, it is able to perform precise distortion compensation on a plurality of wideband signal groups as a whole without adopting broadband frequency-band characteristics for the auxiliary amplifier used to amplify the distortion detected by the distortion detection loop.
Note that in an example of the prior art, the xe2x80x9cfeedforward amplifierxe2x80x9d recited in the publication of unexamined Japanese patent application (Kokai) No. JP-A-2000-236222 (hereinafter referred to as xe2x80x9cReference Document 1xe2x80x9d), broadband characteristics are adopted through the parallel operation of a first auxiliary amplifier that amplifies the distortion components arising on the side of the frequency band lower than the channel band of the input signal, and a second auxiliary amplifier that amplifies the distortion components arising on the side of the frequency band higher than the channel band of the input signal.
Here, upon comparing the prior art recited in Reference Document 1 against the present invention to be presented hereinafter, for example, while the prior art in question has a constitution whereby distortion components in the upside frequency band and distortion components in the downside frequency band arising in an input signal (the signal of one carrier) having a contiguous frequency band are amplified using different auxiliary amplifiers, the present invention differs on the point that its constitution is such that distortion is amplified by a different auxiliary amplifier for each of the various input signal groups arising when input signal groups (e.g., multi-carrier signals) have a plurality of different frequency bands.
In order to achieve the aforementioned object, with the feedforward amplifier according to the present invention, a plurality of groups of signals having different frequency bands are input, a plurality of input signal groups are amplified with a main amplifier in a distortion detection loop and the distortion arising in this main amplifier is detected, and in a distortion compensation loop the distortion detected by the distortion detection loop is removed from the signal amplified by this main amplifier, and at this time, the distortion compensation loop is constituted as follows.
To wit, the distortion compensation loop has the same number of distortion amplification systems as the number of input signal groups, and each distortion amplification system uses an auxiliary amplifier to amplify the distortion for each input signal group that is included in the distortion detected by the distortion detection loop. Moreover, the distortion compensation loop removes the sum total of the distortion from each input signal group amplified by this plurality (to wit, the same number as the number of input signal groups) of distortion amplification systems from the amplified signal from the main amplifier.
Accordingly, the distortion compensation loop is provided with input signal groups corresponding to each input signal group and the same number of distortion amplification systems, and the distortion of each input signal group is amplified by the auxiliary amplifier of each distortion amplification system, so for example, even without adopting broadband frequency band characteristics for the auxiliary amplifiers used to amplify the distortion detected by the distortion detection loop, to wit, even without adopting broadband frequency band characteristics for the auxiliary amplifiers of each distortion amplification system, it is possible to perform precise distortion compensation for the plurality of input signal groups.
Here, the plurality of signal groups each has a different frequency band, and various frequency bands may be used as the frequency bands of each of the signal groups. As a specific example, mutually separated frequency bands may be used as the frequency bands of each signal group.
Various numbers may be used as the number of the plurality of signal groups.
In addition, each of the signal groups may have a frequency bandwidth, for example. Note that a signal with one frequency, for example, may also be used as a signal group according to the present invention, and the present invention also encompasses such an embodiment.
In addition, various compositions may be used as the composition of the distortion detection loop and the composition of the distortion removal loop.
In addition, various amplifiers may be used as the main amplifier. Specific examples of the main amplifier include a common amplifier that is able to amplify signals of a plurality of frequencies all at once.
In addition, the distortion detected by the distortion detection loop includes distortion for each of the respective signal groups, to wit, the distortion arising due to the amplification of the respective signal groups by the main amplifier.
In addition, distortion for each input signal group may include, for example, tertiary distortion arising in the frequency band on the upside (high-frequency side) of each of these input signal groups (upside tertiary distortion), tertiary distortion arising in the frequency band on the downside (low-frequency side) of each of these input signal groups (downside tertiary distortion), etc.
In addition, various degrees of precision may be used as the precision of removal of distortion from the amplified signal from the main amplifier in the distortion compensation loop as long as they are practically effective, to wit, various degrees of precision may be used as the precision of distortion compensation performed in the distortion compensation loop as long as they are practically effective.
In addition, various amplifiers may be used as the auxiliary amplifiers. Specific examples include amplifiers that have characteristics that are good for the amplification of signals in the frequency bands of distortion subject to amplification by each when used as the auxiliary amplifiers for the distortion amplification systems, and even more specific examples include amplifiers wherein the amplitude characteristics and phase characteristics are flat during amplification over the frequency bands of distortion subject to amplification by each.
In addition, the distortion amplification systems used may be ones of various constitutions.
In addition, the sum total of the distortion for each input signal group amplified by all of the distortion amplification systems may be removed from the amplified signal from the main amplifier, or the distortion arising in the main amplifier for all of the input signal groups may be removed from this amplified signal.
Note that in one sample constitution, the distortion compensation loop comprises division means consisting of a divider, for example, that divides the distortion detected by the distortion detection loop among each of the distortion amplification systems, and combining means consisting of a combiner, for example, that obtains the sum total of distortion in each input signal group amplified by the auxiliary amplifier of each distortion amplification system.
In addition, with the feedforward amplifier according to the present invention, as one sample constitution, the distortion amplification systems that have distortion compensation loops are provided with filters that extract the distortion in the input signal groups corresponding to each, and the distortion extracted by these filters is amplified by auxiliary amplifiers.
Accordingly, in each distortion amplification system, the distortion in each input signal group subject to amplification can be extracted by the filter to remove any unnecessary frequency components.
Here, various filters may be used as the filters.
In addition, as specific examples, filters having such characteristics that they extract signals in the frequency band of distortion subject to amplification by the respective distortion amplification systems and in the frequency band of the input signal group corresponding to this distortion may be used as the filters of the respective distortion amplification systems.
In addition, with the feedforward amplifier according to the present invention, as one sample constitution, the various distortion amplification systems having distortion compensation loops are constituted as follows.
To wit, with the various distortion amplification systems having distortion compensation loops, a bandpass filter extracts the distortion in the corresponding respective input signal group, an amplitude changer changes the amplitude of the distortion extracted by the bandpass filter, a phase changer changes the phase of the distortion extracted by the bandpass filter and an auxiliary amplifier amplifies the distortion after its amplitude is changed by the amplitude changer and its phase is changed by the phase changer.
Accordingly, in the various distortion amplification systems, it is possible to exert control by which the amplitude and phase of this distortion is changed at the time of amplification of the respective corresponding input signal groups by the auxiliary amplifiers.
Here, as the mode of changing the amplitude of the distortion, for example, a mode of attenuating the distortion to decrease the amplitude of said distortion, or a mode of amplifying the distortion to increase the amplitude of said distortion may be used.
In addition, a variable attenuator that has the function of attenuating the signal with a variable attenuation, or a variable amplifier that has the function of amplifying the signal with a variable amplification, for example, may be used as the amplitude changer.
In addition, a variable phase shifter that has the function of changing the phase of the signal with a variable amount of phase shift, for example, may be used as the phase changer.
In addition, the order of using the amplitude changer to change amplitude of the distortion and using the phase changer to change the phase of the distortion may be arbitrary. Specifically, a mode wherein the phase of the distortion is changed after changing the amplitude of the distortion may be used, or a mode wherein the amplitude of the distortion is changed after changing the phase of the distortion may be used. Note that at the time of changing the amplitude or phase of the distortion, there may be cases wherein only the amplitude of the distortion is changed, cases wherein only the phase of the distortion is changed, or cases wherein neither the amplitude nor the phase of the distortion is changed.
In addition, with the feedforward amplifier according to the present invention, as one sample constitution, bandpass filters that have flat delay and flat amplification in the frequency band of the extracted distortion is used as the bandpass filter provided in the distortion amplification systems that have distortion compensation loops.
Accordingly, it is possible to maintain the flat delay and flat amplification in filtering by the bandpass filters of each distortion amplification system.
Here, xe2x80x9cflat delay in the frequency band of the extracted distortionxe2x80x9d is defined to be a characteristic wherein the delay arising over the entire range of distortion is the same or on the same level when distortion is extracted in said frequency band.
In addition, xe2x80x9cflat amplification in the frequency band of the extracted distortionxe2x80x9d is defined to be a characteristic wherein the amplification arising over the entire range of distortion is the same or on the same level when distortion is extracted in said frequency band.
In addition, with the feedforward amplifier according to the present invention, as one sample constitution, control is exerted using pilot signals as follows.
To wit, pilot signals are provided for each of the input signal groups, and in this constitution, the number of pilot signals is the same as the number of input signal groups. Moreover, pilot signal synthesizing means is provided for each input signal group for the plurality of input signal groups prior to amplification by the main amplifier provided in the distortion detection loop, and synthesizes the same number of pilot signals as the number of input signal groups. Pilot signal corresponding signal detection means detects signals corresponding to each pilot signal contained within the amplified signal from the main amplifier from which distortion is removed by the distortion compensation loop. Pilot signal control means controls each of the distortion amplification systems having distortion compensation loops based on the results of detection by the pilot signal corresponding signal detection means so that the distortion contained in the amplified signal from the main amplifier with distortion removed by the distortion compensation loop is reduced.
Accordingly, the distortion amplification system corresponding to each of the respective input signal groups is controlled based on pilot signals corresponding to the each of the respective input signal groups, so the distortion contained in the amplified signal from the main amplifier with distortion removed is reduced, so it is possible to perform precise distortion compensation for each input signal group. Note that the distortion contained in the amplified signal from the main amplifier from which distortion is removed is equivalent to the residual distortion not removed by this distortion removal.
Here, as the mode of synthesizing pilot signals for the plurality of input signal groups prior to being amplified by the main amplifier, a mode of synthesizing the pilot signals for the plurality of input signal groups immediately prior to the main amplifier, for example, may be used, or another mode may also be used.
In addition, signals having various frequencies may be used as the pilot signals for each input signal group; it is preferable to use signals having frequencies on the inside of the distortion frequency band of the input signal groups in question, or frequencies nearby on the outside, for example, and it is preferable to use signals having frequencies different from the frequencies constituting the distortion in the input signal groups, for example.
In addition, various signals may be used as the pilot signals.
In addition, as the pilot signal synthesizing means, one that synthesizes pilot signals individually for each input signal group may be used, or one that synthesizes pilot signals for a plurality of input signal groups altogether may also be used.
In addition, as signals corresponding to the pilot signals detected by the pilot signal corresponding signal detection means, signals having the same frequency as the frequency of the various pilot signals, for example, are detected.
In addition, as the pilot signal corresponding signal detection means, one that detects the signals corresponding to each pilot signal individually may be used, or one that detects signals corresponding to a plurality of pilot signals altogether may also be used.
In addition, as the mode of exerting control in order to reduce the distortion contained in the amplified signal from the main amplifier from which distortion is removed by the distortion compensation loop, it is possible to use a mode of control such that the level of the signal detected by the pilot signal corresponding signal detection means becomes small, for example, or it is more preferable to use a mode of control such that the level of this signal is minimized.
In addition, as the mode of controlling the various distortion amplification systems having distortion compensation loops, it is possible to use a mode such that the change in amplification of the amplification changer or the phase change of the phase changer is controlled with respect to the amplification changer or phase changer provided in each distortion amplification system, for example.
In addition, the feedforward amplifier according to the present invention as described above is suitable for use in base station units and relay station units provided in cellular phone systems or Personal Handy phone Systems (PHS) or other mobile communications systems.
As one example, with the base station unit according to the present invention, a feedforward amplifier such as that described above is provided, and multi-carrier or other signals for the mobile station units serving as the other party in communications are amplified by this feedforward amplifier and compensation for the distortion arising at the time of this amplification is performed, and then the amplified signal is transmitted wirelessly to the other party.
Here, various constitutions may be used for the constitution of the mobile communications system, base station unit, relay station unit, mobile station unit or the like.
In addition, various communication protocols can be used as the communication protocol; for example, Code Division Multiple Access (CDMA), Time Division Multiple Access (TDMA), Frequency Division Multiple Access (FDMA) or other communications protocols may be used.